The present invention relates to primary alkaline electrochemical cells having an anode, a separator, an electrolytic solution, and a cathode constructed with an electrolytic manganese dioxide (“EMD”) characterized in that the EMD includes a significant number of large pores. The use of EMD including a high percentage of large pores provides the alkaline battery with improved performance during high discharge rate applications, and also minimizes spring back or rebounce in the die press construction of cathodes.
Small primary electrochemical cells have been commercially available for more than a century. The most successful of these cells have been the cylindrical alkaline batteries of the well known “AAA”, “AA”, “C” and “D” sizes. Most alkaline batteries generally employ an anode, a cathode, a separator, and an aqueous electrolytic solution. The cathode serves as the battery's positive electrode and is typically formed of a mixture of manganese dioxide and carbon or graphite particles. The anode serves as the battery's negative electrode and is generally formed of zinc particles mixed with a gelling agent, usually carboxymethylcellulose. The separator is disposed between the cathode and the anode and typically comprises of a non-woven, inert fabric. The electrolytic solution, which is able to permeate the separator and admix with both the cathode and the anode throughout the battery, is generally a hydroxide solution, such as potassium hydroxide, and provides a path for the transfer of charged ions between the cathode and the anode when they are connected through an external load.
The current generated by an electrochemical cell is directly related to the total surface area of the electrodes and inversely related to the distance between them. Since commercial cells are primarily fixed to the “AAA”, “AA”, “C” and “D” size, it has been desirable to attempt to increase the capacity of the cell by increasing the surface area of the electrode active material and by packing greater amounts of the active material into the cell. To increase the current generated, therefore, prior art batteries often employ cathodes which are constructed of low porosity manganese dioxide.
This approach has practical limitations. For example, if the porosity of the active material is too low, the rate of electrochemical reaction during discharge is generally low, thereby providing a relatively slow rate of discharge of the battery. The dense packing of active material may also cause the cell to become polarized. This is especially true with cells that are exposed to high current drain rates. Polarization of such cells limits the mobility of ions within the electrodes' active material and within the electrolyte solution.
A number of different alkaline cells have been designed in an attempt to maximize power output while avoiding the problems discussed above. Most of these designs have involved modifications to at least one of the four primary elements, namely, the anode, cathode, separator or electrolyte solution.
With respect to cathodes, some design modifications have involved increasing the overall porosity of the manganese dioxide in the cathode. For example, U.S. Pat. No. 5,489,493 to Urry discloses a cathode comprised of manganese dioxide in which the manganese dioxide is a mixture of high porosity manganese dioxide and low porosity manganese dioxide, defining an overall percentage of porosity by weight. This approach also has limitations. An overall porosity rate can be achieved in a number of ways, with a number of different types of materials. The porosity rate can be achieved, for example, with a material having a large number of small pores. Alternatively, the porosity rate can be achieved with a material having a mixture of small and large pores. As described below, the effects on discharge rate are not the same under these two defined circumstances. Therefore, batteries constructed with cathodes with equal overall porosity can behave very differently. In particular, the resultant discharge characteristics can vary significantly from battery to battery, thereby making it difficult to predict operating characteristics and failure times or rates.
Furthermore, highly porous manganese dioxide comprising a large number of small holes can be problematic in the manufacturing process. Cathodes are typically produced by means of a die press process. A measured amount of a cathode mix is added to a ring shaped die set, and the cathode mix is formed into a ring. When highly porous material is employed in producing the ring, “spring back” or rebounce is typically a problem. As the die press is removed from the cathode, compressed air trapped in the pores tends to expand. The expanding air can cause the cathode to expand beyond the required diameter for insertion into the cylindrical container or “can” of the battery, thereby causing waste and an associated loss of time and efficiency.